LTG Cz Research Articles

Growth and spectroscopic investigations of Na2W2O7 single crystals doped with cerium and chromium ions

Alkaline tungstate crystals are promising scintillation materials for rare events search, especially for WIMP investigations. Na2W2O7 single crystals doped with 1.0 mol.% chromium and cerium ions were grown from melt by low-thermal-gradient Czochralski technique (LTG Cz) developed at NIIC SB RAS (Novosibirsk, Russia) for obtaining large oxide scintillating crystals of high optical quality. Obtained doped Na2W2O7 single crystals 25 mm in length and 30 mm in diameter. The excitation and emission spectra of the photoluminescence, as well as the decay curves in the temperature range 77—300 K were measured and colorimetric parameters were estimated. Comparative studies of the photoluminescence properties of undoped and doped with Ce3+ and Cr3+ ions Na2W2O7 crystals in the temperature range of 77—300 K have been carried out. It was shown that while the introduction of cerium and chromium ions in the Na2W2O7 crystal lattice didn¢t significantly effect the colorimetric parameters and optical properties, it did increase mechanical hardness of the crystals and decreased the tendency to cracking along cleavage planes. The emission color chromaticity coordinates, сorrelated color temperature and color purity for Na2W2O7:Ce and Na2W2O7:Cr crystals were calculated. The lifetimes of photoluminescence were calculated and a strong temperature dependence of the lifetimes in the range 77—300 K was shown.

Thermodynamic test of Li2WO4 single crystals with low molybdenum doping: Enthalpy, interconnections, heat capacity

The thermodynamic test of Li2WO4 single crystals doping by low molybdenum content with special emphasis on investigation of interconnections, enthalpy, heat capacity to improve device portfolio was performed. We reduced doping content in 7 times for Li2W1-xMoxO4 in this investigation. The formation enthalpy of Li2WO4 was estimated using interconnections. The heat capacity was measured for Li2W1-0.025Mo0.025O4 single crystal (T = 320–997 K) by differential scanning calorimetry. There were no phase transitions of Li2W1-0.025Mo0.025O4 single crystal that made single crystals with low doping by molybdenum promising for application. The single crystal of lithium tungstate doped by 1.25% molybdenum (content was Li2W0.9875Mo0.0125O4) was grown by LTG CZ for the first time. The reaction enthalpies of Li2W0.9875Mo0.0125O4 single crystal were investigated by reaction calorimetry. The interconnections of formation enthalpies and lattice enthalpies of Li2W1-xMoxO4 in the range (x = 1–0.0125) from x were established. It indicated the growth of phenakite structure in the range. It was shown that under conditions of low temperature gradients it was possible to grow perfect single crystals of lithium tungstate with low doping of molybdenum.

Formation enthalpy and thermodynamic functions of Li2W1-xMoxO4 single crystals (x = 0.025 and 0.05)

A single crystal of lithium tungstate doped by 2.5% molybdenum was grown by low-temperature-gradient Czochralski technique (LTG CZ) with weight control for the first time. The solution enthalpy of Li2W0.975Mo0.025O4 was measured by solution calorimetry in aqueous KOH solvent. Based on experimental data obtained and reference values, the standard formation enthalpy of Li2W0.975Mo0.025O4 single crystal was calculated as following: ΔfH0 (Li2W0.975Mo0.025O4, s, 298.15 K) = − 1599.77 ± 1.88 kJ mol−1. The dependence of lattice enthalpy for Li2W1-xMoxO4 single crystals on tolerance factor for x = 1; 0.15; 0.1; 0.05; 0.025 was constructed. It was shown that dependence of lattice enthalpy on tolerance factor was linear. For low doping of molybdenum, a relation that connected the lattice energy in Li2WO4-Li2MoO4 system with molybdenum content was derived as following: U = A + B · xMo · rMo. The heat capacity of Li2W0.95Mo0.05O4 single crystal was measured in two temperature ranges: (193–370) K; (320–915) K. Heat capacity in both temperature ranges was well described by cubic polynomials. It was shown that there were no phase transitions in both temperature ranges. It is important from a practical point of view for promising application of investigated compounds.

Li2100deplMoO4 crystals grown by low-thermal-gradient Czochralski technique

For the first time, Li2MoO4crystals depleted in molybdenum-100 (100Mo) isotope were grown by Czochralski technique under low thermal gradient conditions (LTG Cz) that made it possible to obtain large crystals of given geometry and required quality. Depleted lithium molybdate crystals Li2deplMoO4 are excellent candidates for creating cryogenic scintillating bolometers with weight of up to 0.3 kg. Such bolometers can be used to reduce the internal radiation background of detectors and to increase sensitivity in physics and astrophysics experiments for rare events search. The grown Li2100deplMoO4 crystal was compared with the crystal samples of natural and enriched in 100Mo isotope in regard of their response to pulsed X-ray irradiation and Raman spectra.

Optical Investigation of Eu3+ Doped Bi12GeO20 (BGO) Crystals

The spectroscopic properties of Eu3+ doped Bi12GeO20 (BGO) sillenite bulk crystals that were grown by the low-thermal-gradient Czochralski technique (LTG Cz) were investigated. The absorption spectra and the emission properties have been measured at room temperature (300 K) and at 10 K. Luminescence was observed both due to the direct Eu3+ ion excitation, as well as under UV excitation due to the energy transfer between Bi3+ and Eu3+ ions. Bi3+ → Eu3+ energy transfer mechanisms in Eu3+:BGO doped host were investigated. The Ωλ parameters, as well as radiative lifetimes, were calculated based upon the Judd-Ofelt formalism. The branching ratios and electric dipole transition probabilities were also determined, based upon the obtained experimental results. Luminescence has been observed from the 5D0,1,2 levels of Eu3+, with emissions from the 5D0 level being the strongest. The strongest observed luminescence band corresponds to the 5D0 → 7F0 transition at 578.7 nm. Reasons for the strong presence of the theoretically forbidden 5D0 → 7F0 emission were investigated.

Thermodynamic characteristics of Li2MoO4, Li2W0.85Mo0.15O4 single crystals and stability direction for alkali molybdates

Single crystals of Li2MoO4 and Li2W0.85Mo0.15O4, as perspective materials to search for neutrino-less double beta decay, have been grown by low-temperature-gradient Czochralski technique (LTG CZ). The standard formation enthalpies for single crystals of Li2MoO4 and Li2W0.85Mo0.15O4 have been determined by two methods: (i) solution calorimetry in KOH (ΔfH0 (Li2MoO4, s, 298.15 K) = −(1521.9 ± 1.9) kJ mol−1) and (ii) high-temperature reaction calorimetry in sodium molybdate melt (3Na2O · 4MoO3) (ΔfH0 (Li2MoO4, s, 298.15 K) = −(1529.1 ± 7.3) kJ mol−1; ΔfH0 (Li2W0.85Mo0.15O4, s, 298.15 K) = −(1597.8 ± 4.3) kJ mol−1). The measured solution enthalpies of Li2CO3, MoO3, WO3, Li2MoO4, Li2W0.85Mo0.15O4 in 3Na2O · 4MoO3 melt and solution enthalpies of Li2CO3, MoO3, Li2MoO4 in KOH have been used to calculateΔfH0. The thermodynamic stability of A2MoO4 (A = Li, Na, K, Rb, Cs) with respect to simple oxides mixture has been determined. It has been found that the relative thermodynamic stability is increased from Li2MoO4 up to Cs2MoO4. This fact seems to be important from practical point of view. The lattice enthalpies for A2MoO4 compounds have been calculated using the Born-Haber cycle. The dependences of lattice enthalpies for A2MoO4 on structural parameters have been constructed, which allow one to predict the thermodynamics for compounds not investigated.

Blocking of dislocation propagation by faceted solid liquid interface during Ge crystal growth by the low thermal gradient Czochralski technique

• Study of initial stage of Ge crystal growth by Low Thermal Gradient Cz technique. • Ge crystal growth in 〈1 1 1〉 with faceted interface blocks dislocation propagation. • Ge crystal growth in 〈1 0 0〉 with rounded interface provides low dislocation density. The initial stage of germanium crystal growth by the low thermal gradient Czochralski technique of pulling from the melt (LTG CZ) has been studied in a series of experiments, in which the processes have been interrupted after pulling 6 mm in dia., 70–80 mm in length crystal rod and then up to 20 mm long a cone part. The axial gradients estimated from computer modelling were 2–2.5 K/cm. Crystals pulled in 〈1 1 1〉 direction with totally faceted interface were dislocation-free if facet formation was not disturbed. Crystals pulled along 〈1 0 0〉 under identical conditions, had rounded interface due to thermal roughens of (1 0 0) faces. In that case, the dislocation density was low, but not below ~10 2 cm −2 .

Na2Mo2O7 scintillating crystals: Growth, morphology and optical properties

For the first time Na2Mo2O7 crystals were grown by Czochralski technique in low-thermal-gradient conditions (LTG Cz), which allowed obtaining crystals of record size (100 (D) × 45 (L) mm3) and optical quality. Influence of growth rate and seed direction on crystal morphology was studied, optimal growth conditions were determined. Three cleavage planes: (0 1 0), (0 0 1) and (0 2 1), were detected. Different characterizations of Na2Mo2O7 crystal samples, such as: transparency, photoluminescence, thermostimulated luminescence and Raman spectra were studied.

Calibration and irradiation study of the BGO background monitor for the BEAST II experiment

Beam commissioning of the SuperKEKB collider began in 2016. The Beam Exorcism for A STable experiment II (BEAST II) project is particularly designed to measure the beam backgrounds around the interaction point of the SuperKEKB collider for the Belle II experiment. We develop a system using bismuth germanium oxide (BGO) crystals with optical fibers connecting to a multianode photomultiplier tube (MAPMT) and a field-programmable gate array (FPGA) embedded readout board for monitoring the real-time beam backgrounds in BEAST II. The overall radiation sensitivity of this system is estimated to be (2.20±0.26)×10−12 Gy/ADU (analog-to-digital unit) with the standard 10 m fibers for transmission and the MAPMT operating at 700 V. Our γ-ray irradiation study of the BGO system shows that the exposure of BGO crystals to 60Co γ-ray doses of 1 krad has led to immediate light output reductions of 25–40%, and the light outputs further drop by 30–45% after the crystals receive doses of 2–4 krad. Our findings agree with those of the previous studies on the radiation hard (RH) BGO crystals grown by the low thermal gradient Czochralski (LTG Cz) technology. The absolute dose from the BGO system is also consistent with the simulation, and is estimated to be about 1.18 times the equivalent dose. These results prove that the BGO system is able to monitor the background dose rate in real time under extreme high radiation conditions. This study concludes that the BGO system is reliable for the beam background study in BEAST II.

Recent progress in oxide scintillation crystals development by low-thermal gradient Czochralski technique for particle physics experiments

Modern particle physics experiments call for high performance scintillation detectors with unique properties: radiation-resistant in high energy and astrophysics, highly radiopure, containing certain elements or enriched isotopes in astroparticle physics. The low-thermal gradient Czochralski (LTG CZ) crystal growth technique provides excellent quality large volume radiopure crystal scintillators. Absence of thermoelastic stress in the crystal and overheating of the melt in the LTG CZ method is particularly significant in production of crystalline materials with strong thermal anisotropic properties and low mechanical strength, with a very high yield of crystalline boules and low losses of initial charge, crucially important in production of crystal scintillators from enriched isotopes for double beta decay experiments. Here we discuss progress in development of the well known scintillators (Bi4Ge3O12 (BGO), CdWO4, ZnWO4, CaMoO4, PbMoO4), as well as R{&}D of new materials (ZnMoO4, Li2MoO4, Na2Mo2O7) for the next generation experiments in particle physics.

The electronic structure of Pb2MoO5: First-principles DFT calculations and X-ray spectroscopy measurements

The electronic structure of lanarkite-type Pb2MoO5 was explored by experimental and theoretical methods. The XPS and XES measurements were carried out at the cleaved atomic-smooth (201̅) surface prepared from the Pb2MoO5 crystal grown by the LTG Cz technique. The ab initio calculations of total and partial densities of states of the atoms constituting Pb2MoO5 were obtained employing the full potential linearized augmented plane wave (FP-LAPW) method. The principal contributors to the valence band of Pb2MoO5 are the O 2p states which contribute predominantly at the top and in the central portions of the valence band. The Pb 6s and Mo 4d states are among other principal contributors: their main contributions occur at the bottom and in the central portion of the valence band, respectively. Our FP-LAPW data reveal that the Pb2MoO5 compound is an indirect-gap material: the valence band maximum and conduction band minimum occur at the X and M points, respectively. The XPS core-level and valence-band spectra were measured for pristine and Ar+ ion-irradiated Pb2MoO5(201̅) surfaces and the measurements reveal high chemical stability of the Pb2MoO5(201̅) cleaved surface in reference to the 3.0keV Ar+ ion-irradiation.

Low Thermal Gradient Czochralski growth of large CdWO4 crystals and electronic properties of (010) cleaved surface

The crystal growth of large high-quality inclusion-free CdWO4 crystals, 110 mm in diameter and mass up to 20kg, has been carried out by the Low Thermal Gradient Czochralski (LTG Cz) technique. The high-purity CdWO4(010) surface has been prepared by cleavage and high structural quality of the surface has been verified by RHEED, revealing a system of Kikuchi lines. The chemical state and electronic structure of the surface have been studied using X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). The total and partial densities of states of the CdWO4 tungstate were calculated employing the first-principles full potential linearized augmented plane wave (FP-LAPW) method. The results indicate that the principal contributors to the valence band of CdWO4 are the Cd 4d, W 5d and O 2p states which contribute mainly at the bottom, in the central portion and at the top of the valence band, respectively, with also significant contributions of the mentioned states throughout the whole CdWO4 valence-band region. With respect to the occupation of the O 2p states, the results of the FP-LAPW calculations are confirmed by comparison on a common energy scale of the XPS valence-band spectrum and the XES band representing the energy distribution of the O 2p states in this compound. Additionally, the FP-LAPW data allow us to conclude that the CdWO4 tungstate is a non-direct semiconductor.

The low thermal gradient Czochralski crystal growth and microstructural properties of a Pb2MoO5(20−1) cleaved surface

High-quality Pb2MoO5 crystals have been grown by the LTG Cz method.

Optimization of the crystallization rate in growing Bi4Ge3O12 crystals by the LTG CZ technique

The influence of the growth rate on the shaping and quality of Bi4Ge3O12 (BGO) crystals during their growth by the low-thermal-gradient Czochralski technique has been studied. Several series of BGO crystals have been grown with the rate varying from 0.3 to 15 mm/h. The limited growing rates at which the crystal quality is not deteriorated are established. The results of the study make it possible to increase the growth rate in commercial systems by a factor of 1.5–2 and thus significantly increase the growth efficiency.

Growth of Na<sub>2</sub>W<sub>2</sub>O<sub>7</sub> Single Crystals as Possible Optical Host Material

High-quality Na2W2O7 single crystals have been grown by low temperature gradient Czochralski technique (LTG Cz). The phase purity of grown crystals has been verified by XRD analysis. A set of oxide compositions based on Na2W2O7 and doped with Ce and special additions for charge compensation has been prepared by solid state synthesis. The formation of Na2W2O7-based solid solutions is found at as high Ce content as ~ 5 mol%.

The low thermal gradient CZ technique as a way of growing of dislocation-free germanium crystals

This paper considers the possibility of growth of dislocation-free germanium single crystals. This is achieved by reducing the temperature gradients at the level of ~1K/cm and lower. Single germanium crystals 45–48mm in diameter with a dislocation density of 102cm−2 were grown by a Low Thermal Gradient Czochralski technique (LTG CZ).

Growth of CdWO4 crystals by the low thermal gradient Czochralski technique and the properties of a (010) cleaved surface

The high-quality CdWO4 crystal of 80–90mm in diameter and 180–200mm long has been grown by Low Thermal Gradient Czochralski technique (LTG Cz). Large area atomically flat CdWO4(010) substrates have been prepared by cleavage. The CdWO4(010) surface is stable in the air up to 600°C. At higher temperatures, the precipitation of WO3 and W19O55 oxides has been detected by RHEED.

Effect of heat shield on the shape of the solid–liquid interface and temperature field in the BGO-eulithine LTG Cz growth

In an effort to eliminate the strong deflection of the solid–liquid interface (SLI) toward a melt, caused by specular (Fresnel) reflection of heat radiation at the crystal surface, we have performed an experimental and numerical study of the growth of Bi 4Ge 3O 12 crystals in the presence of a thermal shield located directly above the crystal shoulder. A platinum shield of a conic shape was used in experiment, while in simulation the shape and radiative properties of the shield are varied. Both experiment and simulation showed that although the shield appreciably decreases the deflection of the solid–liquid interface, increases the size of the facets and diminishes the density of gas inclusions at the initial stage of the growth, the cardinal decrease of the deflection of the interface did not happen.

The effect of radiation damage on optical and scintillation properties of BGO crystals grown by the LTG Cz technique

BGO crystals grown by the low-thermal-gradient Czochralski technique (LTG Cz) exhibit two distinct types of behavior upon radiation damage and recovery. The crystals termed as of L-type remain colorless after γ-radiation doses as high as 10 Mrad. As the irradiation dose increases the scintillation light output shows a weak monotonous degradation to 15–25%, saturating at around several hundreds krad doses. The crystals termed as of N-type attain yellow coloration after irradiation. The light output drops abruptly for 35–50% as early as after 1 krad and does not change further on. The present work is devoted to the study of radiation damage effects, self-recovery, optically stimulated recovery and thermo-stimulated current in the L- and N-type BGO crystals produced by LTG Cz.

Progress in growth of large sized BGO crystals by the low-thermal-gradient Czochralski technique

Recent results of the application of the low-thermal-gradient version of the Czochralski technique (LTG Cz) for growth of top quality, water-transparent, radiation hard large sized Bi 4Ge 3O 12 (BGO) crystals is considered. Boules up to 130 mm in diameter and 400 mm in length are currently growing. At λ=480 nm, the attenuation length of the material in the main part of the boule reaches 10 m or more.